Wavelength-division multiplexing (WDM) technology has become ubiquitous in telecom and datacom networks because it provides a way to satisfy an ever-increasing demand for information bandwidth. In a WDM network, multiple individual optical signals, each carried on a different wavelength, are combined into a composite signal that can be transmitted through the optical fibers of the network with low loss. Historically, aggregate network bandwidth was increased simply by adding more wavelengths and/or transmitting the optical signals at higher data rates.
In recent decades, however, practical and technological barriers have made it increasingly difficult to increase network bandwidth in this manner. As a result, there has been a push toward agile all-optical WDM networks, which enables exploitation of previously underutilized network capacity by reconfiguring the network as bandwidth demand changes. In an optically agile WDM network, the path of an optical signal (single- or multi-wavelength) can be changed without having to convert it from the optical domain into the electrical domain and back again. Path reconfiguration is performed via one or more optical-circuit switches that redirect the photons in an optical signal as desired. The development of practical, fast, optical-circuit switches, therefore, has become a critical aspect of modern high-capacity communications networks.
Prior-art optical-circuit switches include white-light optical cross-connects (i.e., OXCs—cross connects that operate in a wavelength-independent manner), wavelength-selective cross-connects (WXCs—cross-connects whose operation is wavelength dependent), reconfigurable optical add-drop multiplexers (ROADMs—switches that can switch in or out one or more wavelength signals in a composite WDM signal), low-port count switches (e.g., cross-bar switches, 1×2 switches, etc.), and the like. The development of practical systems suitable for widespread deployment in such applications—particularly, large-port-count OXCs, has proven challenging.
Large optical switch fabrics suitable for use in white-light cross connects have been demonstrated using MEMS technology. In such switch fabrics, light beams carrying optical signals are steered from any of M input ports to any of N output ports. The light beams are directed through a three-dimensional free-space volume via opposing arrays of MEMS mirrors, each of which can be controllably tilted in two axes. Representative MEMS-based OXCs are disclosed, for example, by J. Kim, et al., in “1100×1100 port MEMS-based optical cross-connect with 4-dB maximum loss,” IEEE Photonics Technology Letters, vol. 15, no. 11, pp. 1537-1539, Nov.2003. The response time of such switches, however, is limited to about 10 milliseconds while faster switching speeds are required in many applications.
Silicon-core-based planar-lightwave-circuit technology (referred to herein as “silicon photonics”) is seen as an attractive alternative platform to MEMS mirror arrays for high-port-count optical switches. Planar Lightwave Circuits (PLCs) are optical systems comprising one or more waveguides integrated on the surface of a substrate, wherein the waveguides can be combined to provide complex optical functionality. In silicon photonics, these “surface waveguides” include a central core of silicon (or polysilicon) that is typically surrounded by an outer cladding of a material having a refractive index that is lower than that of silicon, such as silicon dioxide, air, or a combination of both. As a result, a light signal propagating through the core is guided along the length of the waveguide by internal reflection at the interface between the silicon and silicon dioxide.
Because the difference between the refractive indices of silicon and silicon dioxide is large, the light propagating through the waveguide is tightly confined to the core material. As a result, a silicon-photonics-based PLC substrate can include a large number of densely packed surface waveguides having tight bends. Examples of silicon-photonics-based OXCs having port counts of 4×4 and 8×8 are disclosed by B. G. Lee, et al., in “Monolithic Silicon Integration of Scaled Photonic Switch Fabrics, CMOS Logic, and Device Driver Circuits,” J Lightwave Technol., vol. 32, no. 4, pp. 743-751, Feb. 2014 and K. Suzuki, et al., in “Ultra-compact 8×8 strictly-non-blocking Si-wire PILOSS switch,” Optics Express, vol. 22, no. 4, p. 3887, Feb. 2014. Unfortunately, these switches comprise large numbers of cascaded 1×2 or 2×2 switches, each of which exhibits significant optical loss. For example, such N×N switch fabrics require N/2 stages of 2×2 switches. As a result, the cumulative optical loss along an optical path through the switch fabric is unacceptably high for many applications.
Another prior-art approach to silicon-photonics-based switching that has the potential to exhibit lower loss is disclosed by L. Chen, et al., in “Compact, low-loss and low-power 8×8 broadband silicon optical switch,” Optics Express, vol. 20, no. 17, pp. 18977-18985, Aug. 2012. In this approach, which is based on a concept referred to as “switch and select,” an N×N switch fabric includes 1×N switches, N×N passive interconnects, and N×2 selectors. Unfortunately, such a switch fabric still includes 2*N*N waveguides and an optical signal path through such the switch fabric encompasses 2*log 2N switches. The maximum number of crossings in the waveguide interconnect area is (N2−1). As a result, the aggregate optical loss for an optical signal propagating through the switch fabric remains quite high.
In similar fashion, an optical switch based on mechanically active silicon-photonics waveguides is disclosed by Akihama and Hane in “Single and multiple optical switches that use freestanding silicon nanowire waveguide couplers, Light: Sci. Appl., Vol. 1, published online (2012), wherein a plurality of 1×2 surface -waveguide-based optical switches are cascaded to form a 1×6 optical switch. Each optical switch includes first and second surface waveguides, the first waveguide having an input and a first output and the second waveguide having a second output. A portion of one waveguide is freestanding and movable relative to a portion of the other waveguide, where the two waveguide portions collectively define a switchable directional coupler. The movable waveguide portion is moved laterally into and out of proximity with the other waveguide by a MEMS-based lateral comb-drive actuator. When the two waveguide portions are not in close proximity, a light signal injected at the input port stays in the first waveguide and propagates to the first output. When the movable waveguide portion is brought into close proximity with the second waveguide, the light signal is coupled into the second waveguide at the directional coupler and, as a result, propagates to the second output.
Unfortunately, this approach has several drawbacks. First, the freestanding waveguide portion must be quite long. Length is needed to provide enough flexibility to the waveguide to enable its motion between its coupled and uncoupled positions, as well as to mitigate bending losses at bends in the waveguide that arise as the waveguide is moved. As a result, such an optical switch requires a great deal of chip real estate, which leads to high cost.
Second, due to the cascade arrangement of the switch, different paths through the optical switch include markedly different numbers of directional couplers. As a result, such optical switches are subject to path-dependent losses that vary considerably from path to path. Path-dependent-loss variation can lead to complicated issues in systems employing such switches. Further, the need to limit the variation of path-dependent loss can place an upper bound on the size of such systems.
A practical, fast, low-cost, low-loss optical switching technology suitable for use in switching elements of low- and high-port counts remains, as yet, unavailable in the prior art.